Exercise-training is widely used in the rehabilitation of patients with ischemic heart disease (ISCHD). However, there is no convincing evidence that training improves myocardial blood supply, function, or electrical stability in individuals with ISCHD. We propose to: (1) Determine whether or not a prolonged program of endurance exercise (11 months) can improve myocardial blood supply in patients with ISCHD. Myocardial blood supply will be evaluated by means of (a) Thallium 201 myocardial imaging at rest and following maximum attainable exercise, and (b) exercise electrocardiography with determination of the HR x BP product threshold for exercise-induced ECG evidence of myocardial ischemia. (2) Evaluate the effects of an 11 month program of endurance exercise on myocardial performance assessed by means of (a) ECG gated 99mTc-albumin radionuclide cineangiograms, (b) echocardiography, (c) systolic time intervals, and (d) cardiac output measurements at rest and during exercise using the CO2 rebreathing method. (3) Determine whether exercise-training can protect patients with ISCHD against ventricular arrhythmias. Electrical stability of the heart will be evaluated by means of automated analysis of continuous 24 hr portable ECG recordings, and of ECG's obtained during and after exercise-testing, to determine the frequency of premature ventricular depolarizations.